Perhaps I am not the best person to offer perspective since relations with my in-laws are far from perfect, but here's my two cents.

If your mother-in-law wants you to go to family therapy, it means she actually does considers you part of the family. That's good, right? Could it be that she wants to have a better relationship with you and your husband, but she doesn't know how to make that happen herself? Could this be her way of trying to meet you partway?

The idea of my own M-I-L suggesting family therapy is so unfathomable that it's difficult for me to imagine. But if she really wanted it, I would at least try it on the condition that the choice of therapist must be unanimous - everyone must feel comfortable. Good therapy is never easy, but a skilled practitioner will ensure that the playing field is level and no one feels ganged up upon.

Although I have not been to family therapy, my husband and I did go to marriage therapy two years ago. When I walked in to the first appointment, I was convinced that our problems were 100% attributed to my husband. I'm pretty sure he felt the same way about me. We had a really great therapist who helped us realize how our own stuff can get in the way of perceiving interactions and situations, and that both of us had a role in creating the misunderstandings and dynamic that existed.

All I'm saying is that maybe if you give therapy a shot, you will learn new things about yourself and perhaps improve relations with your in-laws. What have you got to lose?